/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.j2objc.annotations.Weak;
import com.google.j2objc.annotations.WeakOuter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.locks.ReentrantLock;
import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;

/**
 * The concurrent hash map implementation built by {@link MapMaker}.
 *
 * <p>
 * This implementation is heavily derived from revision 1.96 of
 * <a href="http://tinyurl.com/ConcurrentHashMap">ConcurrentHashMap.java</a>.
 *
 * @param <K> the type of the keys in the map
 * @param <V> the type of the values in the map
 * @param <E> the type of the {@link InternalEntry} entry implementation used internally
 * @param <S> the type of the {@link Segment} entry implementation used internally
 * @author Bob Lee
 * @author Charles Fry
 * @author Doug Lea ({@code ConcurrentHashMap})
 */
// TODO(kak/cpovirk): Consider removing @CanIgnoreReturnValue from this class.
@GwtIncompatible
class MapMakerInternalMap<K, V, E extends MapMakerInternalMap.InternalEntry<K, V, E>, S extends MapMakerInternalMap.Segment<K, V, E, S>>
        extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable {

    /*
     * The basic strategy is to subdivide the table among Segments, each of which itself is a
     * concurrently readable hash table. The map supports non-blocking reads and concurrent writes
     * across different segments.
     *
     * The page replacement algorithm's data structures are kept casually consistent with the map.
     * The ordering of writes to a segment is sequentially consistent. An update to the map and
     * recording of reads may not be immediately reflected on the algorithm's data structures. These
     * structures are guarded by a lock and operations are applied in batches to avoid lock
     * contention. The penalty of applying the batches is spread across threads so that the
     * amortized cost is slightly higher than performing just the operation without enforcing the
     * capacity constraint.
     *
     * This implementation uses a per-segment queue to record a memento of the additions, removals,
     * and accesses that were performed on the map. The queue is drained on writes and when it
     * exceeds its capacity threshold.
     *
     * The Least Recently Used page replacement algorithm was chosen due to its simplicity, high hit
     * rate, and ability to be implemented with O(1) time complexity. The initial LRU implementation
     * operates per-segment rather than globally for increased implementation simplicity. We expect
     * the cache hit rate to be similar to that of a global LRU algorithm.
     */

    // Constants

    /**
     * The maximum capacity, used if a higher value is implicitly specified by either of the
     * constructors with arguments. MUST be a power of two no greater than {@code 1<<30} to ensure
     * that entries are indexable using ints.
     */
    static final int MAXIMUM_CAPACITY = Ints.MAX_POWER_OF_TWO;

    /** The maximum number of segments to allow; used to bound constructor arguments. */
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /** Number of (unsynchronized) retries in the containsValue method. */
    static final int CONTAINS_VALUE_RETRIES = 3;

    /**
     * Number of cache access operations that can be buffered per segment before the cache's recency
     * ordering information is updated. This is used to avoid lock contention by recording a memento
     * of reads and delaying a lock acquisition until the threshold is crossed or a mutation occurs.
     *
     * <p>
     * This must be a (2^n)-1 as it is used as a mask.
     */
    static final int DRAIN_THRESHOLD = 0x3F;

    /**
     * Maximum number of entries to be drained in a single cleanup run. This applies independently
     * to the cleanup queue and both reference queues.
     */
    // TODO(fry): empirically optimize this
    static final int DRAIN_MAX = 16;

    static final long CLEANUP_EXECUTOR_DELAY_SECS = 60;

    // Fields

    /**
     * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose
     * the segment.
     */
    final transient int segmentMask;

    /**
     * Shift value for indexing within segments. Helps prevent entries that end up in the same
     * segment from also ending up in the same bucket.
     */
    final transient int segmentShift;

    /** The segments, each of which is a specialized hash table. */
    final transient Segment<K, V, E, S>[] segments;

    /** The concurrency level. */
    final int concurrencyLevel;

    /** Strategy for comparing keys. */
    final Equivalence<Object> keyEquivalence;

    /** Strategy for handling entries and segments in a type-safe and efficient manner. */
    final transient InternalEntryHelper<K, V, E, S> entryHelper;

    /**
     * Creates a new, empty map with the specified strategy, initial capacity and concurrency level.
     */
    private MapMakerInternalMap(MapMaker builder, InternalEntryHelper<K, V, E, S> entryHelper) {
        concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);

        keyEquivalence = builder.getKeyEquivalence();
        this.entryHelper = entryHelper;

        int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);

        // Find power-of-two sizes best matching arguments. Constraints:
        // (segmentCount > concurrencyLevel)
        int segmentShift = 0;
        int segmentCount = 1;
        while (segmentCount < concurrencyLevel) {
            ++segmentShift;
            segmentCount <<= 1;
        }
        this.segmentShift = 32 - segmentShift;
        segmentMask = segmentCount - 1;

        this.segments = newSegmentArray(segmentCount);

        int segmentCapacity = initialCapacity / segmentCount;
        if (segmentCapacity * segmentCount < initialCapacity) {
            ++segmentCapacity;
        }

        int segmentSize = 1;
        while (segmentSize < segmentCapacity) {
            segmentSize <<= 1;
        }

        for (int i = 0; i < this.segments.length; ++i) {
            this.segments[i] = createSegment(segmentSize, MapMaker.UNSET_INT);
        }
    }

    static <K, V> MapMakerInternalMap<K, V, ? extends InternalEntry<K, V, ?>, ?> create(MapMaker builder) {
        if (builder.getKeyStrength() == Strength.STRONG && builder.getValueStrength() == Strength.STRONG) {
            return new MapMakerInternalMap<K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>>(
                    builder, StrongKeyStrongValueEntry.Helper.<K, V>instance());
        }
        if (builder.getKeyStrength() == Strength.STRONG && builder.getValueStrength() == Strength.WEAK) {
            return new MapMakerInternalMap<K, V, StrongKeyWeakValueEntry<K, V>, StrongKeyWeakValueSegment<K, V>>(
                    builder, StrongKeyWeakValueEntry.Helper.<K, V>instance());
        }
        if (builder.getKeyStrength() == Strength.WEAK && builder.getValueStrength() == Strength.STRONG) {
            return new MapMakerInternalMap<K, V, WeakKeyStrongValueEntry<K, V>, WeakKeyStrongValueSegment<K, V>>(
                    builder, WeakKeyStrongValueEntry.Helper.<K, V>instance());
        }
        if (builder.getKeyStrength() == Strength.WEAK && builder.getValueStrength() == Strength.WEAK) {
            return new MapMakerInternalMap<K, V, WeakKeyWeakValueEntry<K, V>, WeakKeyWeakValueSegment<K, V>>(builder,
                    WeakKeyWeakValueEntry.Helper.<K, V>instance());
        }
        throw new AssertionError();
    }

    enum Strength {
        STRONG {
            @Override
            Equivalence<Object> defaultEquivalence() {
                return Equivalence.equals();
            }
        },

        WEAK {
            @Override
            Equivalence<Object> defaultEquivalence() {
                return Equivalence.identity();
            }
        };

        /**
         * Returns the default equivalence strategy used to compare and hash keys or values
         * referenced at this strength. This strategy will be used unless the user explicitly
         * specifies an alternate strategy.
         */
        abstract Equivalence<Object> defaultEquivalence();
    }

    /**
     * A helper object for operating on {@link InternalEntry} instances in a type-safe and efficient
     * manner.
     *
     * <p>
     * For each of the four combinations of strong/weak key and strong/weak value, there are
     * corresponding {@link InternalEntry}, {@link Segment}, and {@link InternalEntryHelper}
     * implementations.
     *
     * @param <K> the type of the key in each entry
     * @param <V> the type of the value in each entry
     * @param <E> the type of the {@link InternalEntry} entry implementation
     * @param <S> the type of the {@link Segment} entry implementation
     */
    interface InternalEntryHelper<K, V, E extends InternalEntry<K, V, E>, S extends Segment<K, V, E, S>> {
        /** The strength of the key type in each entry. */
        Strength keyStrength();

        /** The strength of the value type in each entry. */
        Strength valueStrength();

        /** Returns a freshly created segment, typed at the {@code S} type. */
        S newSegment(MapMakerInternalMap<K, V, E, S> map, int initialCapacity, int maxSegmentSize);

        /**
         * Returns a freshly created entry, typed at the {@code E} type, for the given
         * {@code segment}.
         */
        E newEntry(S segment, K key, int hash, @Nullable E next);

        /**
         * Returns a freshly created entry, typed at the {@code E} type, for the given
         * {@code segment}, that is a copy of the given {@code entry}.
         */
        E copy(S segment, E entry, @Nullable E newNext);

        /**
         * Sets the value of the given {@code entry} in the given {@code segment} to be the given
         * {@code
         * value}
         */
        void setValue(S segment, E entry, V value);
    }

    /**
     * An entry in a hash table of a {@link Segment}.
     *
     * <p>
     * Entries in the map can be in the following states:
     *
     * <p>
     * Valid: - Live: valid key/value are set
     *
     * <p>
     * Invalid: - Collected: key/value was partially collected, but not yet cleaned up
     */
    interface InternalEntry<K, V, E extends InternalEntry<K, V, E>> {
        /** Gets the next entry in the chain. */
        E getNext();

        /**
         * Gets the entry's hash.
         */
        int getHash();

        /**
         * Gets the key for this entry.
         */
        K getKey();

        /** Gets the value for the entry. */
        V getValue();
    }

    /*
     * Note: the following classes have a lot of duplicate code. It sucks, but it saves a lot of
     * memory. If only Java had mixins!
     */

    /** Base class for {@link InternalEntry} implementations for strong keys. */
    abstract static class AbstractStrongKeyEntry<K, V, E extends InternalEntry<K, V, E>>
            implements InternalEntry<K, V, E> {
        final K key;
        final int hash;
        final E next;

        AbstractStrongKeyEntry(K key, int hash, @Nullable E next) {
            this.key = key;
            this.hash = hash;
            this.next = next;
        }

        @Override
        public K getKey() {
            return this.key;
        }

        @Override
        public int getHash() {
            return hash;
        }

        @Override
        public E getNext() {
            return next;
        }
    }

    /** Marker interface for {@link InternalEntry} implementations for strong values. */
    interface StrongValueEntry<K, V, E extends InternalEntry<K, V, E>> extends InternalEntry<K, V, E> {
    }

    /** Marker interface for {@link InternalEntry} implementations for weak values. */
    interface WeakValueEntry<K, V, E extends InternalEntry<K, V, E>> extends InternalEntry<K, V, E> {
        /** Gets the weak value reference held by entry. */
        WeakValueReference<K, V, E> getValueReference();

        /**
         * Clears the weak value reference held by the entry. Should be used when the entry's value
         * is overwritten.
         */
        void clearValue();
    }

    @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
    static <K, V, E extends InternalEntry<K, V, E>> WeakValueReference<K, V, E> unsetWeakValueReference() {
        return (WeakValueReference<K, V, E>) UNSET_WEAK_VALUE_REFERENCE;
    }

    /** Concrete implementation of {@link InternalEntry} for strong keys and strong values. */
    static final class StrongKeyStrongValueEntry<K, V>
            extends AbstractStrongKeyEntry<K, V, StrongKeyStrongValueEntry<K, V>>
            implements StrongValueEntry<K, V, StrongKeyStrongValueEntry<K, V>> {
        @Nullable
        private volatile V value = null;

        StrongKeyStrongValueEntry(K key, int hash, @Nullable StrongKeyStrongValueEntry<K, V> next) {
            super(key, hash, next);
        }

        @Override
        @Nullable
        public V getValue() {
            return value;
        }

        void setValue(V value) {
            this.value = value;
        }

        StrongKeyStrongValueEntry<K, V> copy(StrongKeyStrongValueEntry<K, V> newNext) {
            StrongKeyStrongValueEntry<K, V> newEntry =
                    new StrongKeyStrongValueEntry<K, V>(this.key, this.hash, newNext);
            newEntry.value = this.value;
            return newEntry;
        }

        /**
         * Concrete implementation of {@link InternalEntryHelper} for strong keys and strong values.
         */
        static final class Helper<K, V> implements
                InternalEntryHelper<K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>> {
            private static final Helper<?, ?> INSTANCE = new Helper<Object, Object>();

            @SuppressWarnings("unchecked")
            static <K, V> Helper<K, V> instance() {
                return (Helper<K, V>) INSTANCE;
            }

            @Override
            public Strength keyStrength() {
                return Strength.STRONG;
            }

            @Override
            public Strength valueStrength() {
                return Strength.STRONG;
            }

            @Override
            public StrongKeyStrongValueSegment<K, V> newSegment(
                    MapMakerInternalMap<K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>> map,
                    int initialCapacity, int maxSegmentSize) {
                return new StrongKeyStrongValueSegment<K, V>(map, initialCapacity, maxSegmentSize);
            }

            @Override
            public StrongKeyStrongValueEntry<K, V> copy(StrongKeyStrongValueSegment<K, V> segment,
                    StrongKeyStrongValueEntry<K, V> entry, @Nullable StrongKeyStrongValueEntry<K, V> newNext) {
                return entry.copy(newNext);
            }

            @Override
            public void setValue(StrongKeyStrongValueSegment<K, V> segment, StrongKeyStrongValueEntry<K, V> entry,
                    V value) {
                entry.setValue(value);
            }

            @Override
            public StrongKeyStrongValueEntry<K, V> newEntry(StrongKeyStrongValueSegment<K, V> segment, K key, int hash,
                    @Nullable StrongKeyStrongValueEntry<K, V> next) {
                return new StrongKeyStrongValueEntry<K, V>(key, hash, next);
            }
        }
    }

    /** Concrete implementation of {@link InternalEntry} for strong keys and weak values. */
    static final class StrongKeyWeakValueEntry<K, V> extends AbstractStrongKeyEntry<K, V, StrongKeyWeakValueEntry<K, V>>
            implements WeakValueEntry<K, V, StrongKeyWeakValueEntry<K, V>> {
        private volatile WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> valueReference =
                unsetWeakValueReference();

        StrongKeyWeakValueEntry(K key, int hash, @Nullable StrongKeyWeakValueEntry<K, V> next) {
            super(key, hash, next);
        }

        @Override
        public V getValue() {
            return valueReference.get();
        }

        @Override
        public void clearValue() {
            valueReference.clear();
        }

        void setValue(V value, ReferenceQueue<V> queueForValues) {
            WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> previous = this.valueReference;
            this.valueReference =
                    new WeakValueReferenceImpl<K, V, StrongKeyWeakValueEntry<K, V>>(queueForValues, value, this);
            previous.clear();
        }

        StrongKeyWeakValueEntry<K, V> copy(ReferenceQueue<V> queueForValues, StrongKeyWeakValueEntry<K, V> newNext) {
            StrongKeyWeakValueEntry<K, V> newEntry = new StrongKeyWeakValueEntry<K, V>(key, hash, newNext);
            newEntry.valueReference = valueReference.copyFor(queueForValues, newEntry);
            return newEntry;
        }

        @Override
        public WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> getValueReference() {
            return valueReference;
        }

        /**
         * Concrete implementation of {@link InternalEntryHelper} for strong keys and weak values.
         */
        static final class Helper<K, V>
                implements InternalEntryHelper<K, V, StrongKeyWeakValueEntry<K, V>, StrongKeyWeakValueSegment<K, V>> {
            private static final Helper<?, ?> INSTANCE = new Helper<Object, Object>();

            @SuppressWarnings("unchecked")
            static <K, V> Helper<K, V> instance() {
                return (Helper<K, V>) INSTANCE;
            }

            @Override
            public Strength keyStrength() {
                return Strength.STRONG;
            }

            @Override
            public Strength valueStrength() {
                return Strength.WEAK;
            }

            @Override
            public StrongKeyWeakValueSegment<K, V> newSegment(
                    MapMakerInternalMap<K, V, StrongKeyWeakValueEntry<K, V>, StrongKeyWeakValueSegment<K, V>> map,
                    int initialCapacity, int maxSegmentSize) {
                return new StrongKeyWeakValueSegment<K, V>(map, initialCapacity, maxSegmentSize);
            }

            @Override
            public StrongKeyWeakValueEntry<K, V> copy(StrongKeyWeakValueSegment<K, V> segment,
                    StrongKeyWeakValueEntry<K, V> entry, @Nullable StrongKeyWeakValueEntry<K, V> newNext) {
                if (Segment.isCollected(entry)) {
                    return null;
                }
                return entry.copy(segment.queueForValues, newNext);
            }

            @Override
            public void setValue(StrongKeyWeakValueSegment<K, V> segment, StrongKeyWeakValueEntry<K, V> entry,
                    V value) {
                entry.setValue(value, segment.queueForValues);
            }

            @Override
            public StrongKeyWeakValueEntry<K, V> newEntry(StrongKeyWeakValueSegment<K, V> segment, K key, int hash,
                    @Nullable StrongKeyWeakValueEntry<K, V> next) {
                return new StrongKeyWeakValueEntry<K, V>(key, hash, next);
            }
        }
    }

    /** Base class for {@link InternalEntry} implementations for weak keys. */
    abstract static class AbstractWeakKeyEntry<K, V, E extends InternalEntry<K, V, E>> extends WeakReference<K>
            implements InternalEntry<K, V, E> {
        final int hash;
        final E next;

        AbstractWeakKeyEntry(ReferenceQueue<K> queue, K key, int hash, @Nullable E next) {
            super(key, queue);
            this.hash = hash;
            this.next = next;
        }

        @Override
        public K getKey() {
            return get();
        }

        @Override
        public int getHash() {
            return hash;
        }

        @Override
        public E getNext() {
            return next;
        }
    }

    /** Concrete implementation of {@link InternalEntry} for weak keys and strong values. */
    static final class WeakKeyStrongValueEntry<K, V> extends AbstractWeakKeyEntry<K, V, WeakKeyStrongValueEntry<K, V>>
            implements StrongValueEntry<K, V, WeakKeyStrongValueEntry<K, V>> {
        @Nullable
        private volatile V value = null;

        WeakKeyStrongValueEntry(ReferenceQueue<K> queue, K key, int hash,
                @Nullable WeakKeyStrongValueEntry<K, V> next) {
            super(queue, key, hash, next);
        }

        @Override
        @Nullable
        public V getValue() {
            return value;
        }

        void setValue(V value) {
            this.value = value;
        }

        WeakKeyStrongValueEntry<K, V> copy(ReferenceQueue<K> queueForKeys, WeakKeyStrongValueEntry<K, V> newNext) {
            WeakKeyStrongValueEntry<K, V> newEntry =
                    new WeakKeyStrongValueEntry<K, V>(queueForKeys, getKey(), this.hash, newNext);
            newEntry.setValue(value);
            return newEntry;
        }

        /**
         * Concrete implementation of {@link InternalEntryHelper} for weak keys and strong values.
         */
        static final class Helper<K, V>
                implements InternalEntryHelper<K, V, WeakKeyStrongValueEntry<K, V>, WeakKeyStrongValueSegment<K, V>> {
            private static final Helper<?, ?> INSTANCE = new Helper<Object, Object>();

            @SuppressWarnings("unchecked")
            static <K, V> Helper<K, V> instance() {
                return (Helper<K, V>) INSTANCE;
            }

            @Override
            public Strength keyStrength() {
                return Strength.WEAK;
            }

            @Override
            public Strength valueStrength() {
                return Strength.STRONG;
            }

            @Override
            public WeakKeyStrongValueSegment<K, V> newSegment(
                    MapMakerInternalMap<K, V, WeakKeyStrongValueEntry<K, V>, WeakKeyStrongValueSegment<K, V>> map,
                    int initialCapacity, int maxSegmentSize) {
                return new WeakKeyStrongValueSegment<K, V>(map, initialCapacity, maxSegmentSize);
            }

            @Override
            public WeakKeyStrongValueEntry<K, V> copy(WeakKeyStrongValueSegment<K, V> segment,
                    WeakKeyStrongValueEntry<K, V> entry, @Nullable WeakKeyStrongValueEntry<K, V> newNext) {
                if (entry.getKey() == null) {
                    // key collected
                    return null;
                }
                return entry.copy(segment.queueForKeys, newNext);
            }

            @Override
            public void setValue(WeakKeyStrongValueSegment<K, V> segment, WeakKeyStrongValueEntry<K, V> entry,
                    V value) {
                entry.setValue(value);
            }

            @Override
            public WeakKeyStrongValueEntry<K, V> newEntry(WeakKeyStrongValueSegment<K, V> segment, K key, int hash,
                    @Nullable WeakKeyStrongValueEntry<K, V> next) {
                return new WeakKeyStrongValueEntry<K, V>(segment.queueForKeys, key, hash, next);
            }
        }
    }

    /** Concrete implementation of {@link InternalEntry} for weak keys and weak values. */
    static final class WeakKeyWeakValueEntry<K, V> extends AbstractWeakKeyEntry<K, V, WeakKeyWeakValueEntry<K, V>>
            implements WeakValueEntry<K, V, WeakKeyWeakValueEntry<K, V>> {
        private volatile WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> valueReference =
                unsetWeakValueReference();

        WeakKeyWeakValueEntry(ReferenceQueue<K> queue, K key, int hash, @Nullable WeakKeyWeakValueEntry<K, V> next) {
            super(queue, key, hash, next);
        }

        @Override
        public V getValue() {
            return valueReference.get();
        }

        WeakKeyWeakValueEntry<K, V> copy(ReferenceQueue<K> queueForKeys, ReferenceQueue<V> queueForValues,
                WeakKeyWeakValueEntry<K, V> newNext) {
            WeakKeyWeakValueEntry<K, V> newEntry =
                    new WeakKeyWeakValueEntry<K, V>(queueForKeys, getKey(), this.hash, newNext);
            newEntry.valueReference = valueReference.copyFor(queueForValues, newEntry);
            return newEntry;
        }

        @Override
        public void clearValue() {
            valueReference.clear();
        }

        void setValue(V value, ReferenceQueue<V> queueForValues) {
            WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> previous = this.valueReference;
            this.valueReference =
                    new WeakValueReferenceImpl<K, V, WeakKeyWeakValueEntry<K, V>>(queueForValues, value, this);
            previous.clear();
        }

        @Override
        public WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> getValueReference() {
            return valueReference;
        }

        /** Concrete implementation of {@link InternalEntryHelper} for weak keys and weak values. */
        static final class Helper<K, V>
                implements InternalEntryHelper<K, V, WeakKeyWeakValueEntry<K, V>, WeakKeyWeakValueSegment<K, V>> {
            private static final Helper<?, ?> INSTANCE = new Helper<Object, Object>();

            @SuppressWarnings("unchecked")
            static <K, V> Helper<K, V> instance() {
                return (Helper<K, V>) INSTANCE;
            }

            @Override
            public Strength keyStrength() {
                return Strength.WEAK;
            }

            @Override
            public Strength valueStrength() {
                return Strength.WEAK;
            }

            @Override
            public WeakKeyWeakValueSegment<K, V> newSegment(
                    MapMakerInternalMap<K, V, WeakKeyWeakValueEntry<K, V>, WeakKeyWeakValueSegment<K, V>> map,
                    int initialCapacity, int maxSegmentSize) {
                return new WeakKeyWeakValueSegment<K, V>(map, initialCapacity, maxSegmentSize);
            }

            @Override
            public WeakKeyWeakValueEntry<K, V> copy(WeakKeyWeakValueSegment<K, V> segment,
                    WeakKeyWeakValueEntry<K, V> entry, @Nullable WeakKeyWeakValueEntry<K, V> newNext) {
                if (entry.getKey() == null) {
                    // key collected
                    return null;
                }
                if (Segment.isCollected(entry)) {
                    return null;
                }
                return entry.copy(segment.queueForKeys, segment.queueForValues, newNext);
            }

            @Override
            public void setValue(WeakKeyWeakValueSegment<K, V> segment, WeakKeyWeakValueEntry<K, V> entry, V value) {
                entry.setValue(value, segment.queueForValues);
            }

            @Override
            public WeakKeyWeakValueEntry<K, V> newEntry(WeakKeyWeakValueSegment<K, V> segment, K key, int hash,
                    @Nullable WeakKeyWeakValueEntry<K, V> next) {
                return new WeakKeyWeakValueEntry<K, V>(segment.queueForKeys, key, hash, next);
            }
        }
    }

    /** A weakly referenced value that also has a reference to its containing entry. */
    interface WeakValueReference<K, V, E extends InternalEntry<K, V, E>> {
        /**
         * Returns the current value being referenced, or {@code null} if there is none (e.g.
         * because either it got collected, or {@link #clear} was called, or it wasn't set in the
         * first place).
         */
        @Nullable
        V get();

        /** Returns the entry which contains this {@link WeakValueReference}. */
        E getEntry();

        /**
         * Unsets the referenced value. Subsequent calls to {@link #get} will return {@code null}.
         */
        void clear();

        /**
         * Returns a freshly created {@link WeakValueReference} for the given {@code entry} (and on
         * the given {@code queue} with the same value as this {@link WeakValueReference}.
         */
        WeakValueReference<K, V, E> copyFor(ReferenceQueue<V> queue, E entry);
    }

    /**
     * A dummy implementation of {@link InternalEntry}, solely for use in the type signature of
     * {@link #UNSET_WEAK_VALUE_REFERENCE} below.
     */
    static final class DummyInternalEntry implements InternalEntry<Object, Object, DummyInternalEntry> {
        private DummyInternalEntry() {
            throw new AssertionError();
        }

        @Override
        public DummyInternalEntry getNext() {
            throw new AssertionError();
        }

        @Override
        public int getHash() {
            throw new AssertionError();
        }

        @Override
        public Object getKey() {
            throw new AssertionError();
        }

        @Override
        public Object getValue() {
            throw new AssertionError();
        }
    }

    /**
     * A singleton {@link WeakValueReference} used to denote an unset value in a entry with weak
     * values.
     */
    static final WeakValueReference<Object, Object, DummyInternalEntry> UNSET_WEAK_VALUE_REFERENCE =
            new WeakValueReference<Object, Object, DummyInternalEntry>() {
                @Override
                public DummyInternalEntry getEntry() {
                    return null;
                }

                @Override
                public void clear() {}

                @Override
                public Object get() {
                    return null;
                }

                @Override
                public WeakValueReference<Object, Object, DummyInternalEntry> copyFor(ReferenceQueue<Object> queue,
                        DummyInternalEntry entry) {
                    return this;
                }
            };

    /** Concrete implementation of {@link WeakValueReference}. */
    static final class WeakValueReferenceImpl<K, V, E extends InternalEntry<K, V, E>> extends WeakReference<V>
            implements WeakValueReference<K, V, E> {
        @Weak
        final E entry;

        WeakValueReferenceImpl(ReferenceQueue<V> queue, V referent, E entry) {
            super(referent, queue);
            this.entry = entry;
        }

        @Override
        public E getEntry() {
            return entry;
        }

        @Override
        public WeakValueReference<K, V, E> copyFor(ReferenceQueue<V> queue, E entry) {
            return new WeakValueReferenceImpl<K, V, E>(queue, get(), entry);
        }
    }

    /**
     * Applies a supplemental hash function to a given hash code, which defends against poor quality
     * hash functions. This is critical when the concurrent hash map uses power-of-two length hash
     * tables, that otherwise encounter collisions for hash codes that do not differ in lower or
     * upper bits.
     *
     * @param h hash code
     */
    static int rehash(int h) {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        // TODO(kevinb): use Hashing/move this to Hashing?
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    /**
     * This method is a convenience for testing. Code should call {@link Segment#copyEntry}
     * directly.
     */
    // Guarded By Segment.this
    @VisibleForTesting
    E copyEntry(E original, E newNext) {
        int hash = original.getHash();
        return segmentFor(hash).copyEntry(original, newNext);
    }

    int hash(Object key) {
        int h = keyEquivalence.hash(key);
        return rehash(h);
    }

    void reclaimValue(WeakValueReference<K, V, E> valueReference) {
        E entry = valueReference.getEntry();
        int hash = entry.getHash();
        segmentFor(hash).reclaimValue(entry.getKey(), hash, valueReference);
    }

    void reclaimKey(E entry) {
        int hash = entry.getHash();
        segmentFor(hash).reclaimKey(entry, hash);
    }

    /**
     * This method is a convenience for testing. Code should call {@link Segment#getLiveValue}
     * instead.
     */
    @VisibleForTesting
    boolean isLiveForTesting(InternalEntry<K, V, ?> entry) {
        return segmentFor(entry.getHash()).getLiveValueForTesting(entry) != null;
    }

    /**
     * Returns the segment that should be used for a key with the given hash.
     *
     * @param hash the hash code for the key
     * @return the segment
     */
    Segment<K, V, E, S> segmentFor(int hash) {
        // TODO(fry): Lazily create segments?
        return segments[(hash >>> segmentShift) & segmentMask];
    }

    Segment<K, V, E, S> createSegment(int initialCapacity, int maxSegmentSize) {
        return entryHelper.newSegment(this, initialCapacity, maxSegmentSize);
    }

    /**
     * Gets the value from an entry. Returns {@code null} if the entry is invalid,
     * partially-collected or computing.
     */
    V getLiveValue(E entry) {
        if (entry.getKey() == null) {
            return null;
        }
        V value = entry.getValue();
        if (value == null) {
            return null;
        }
        return value;
    }

    @SuppressWarnings("unchecked")
    final Segment<K, V, E, S>[] newSegmentArray(int ssize) {
        return new Segment[ssize];
    }

    // Inner Classes

    /**
     * Segments are specialized versions of hash tables. This subclass inherits from ReentrantLock
     * opportunistically, just to simplify some locking and avoid separate construction.
     */
    @SuppressWarnings("serial") // This class is never serialized.
    abstract static class Segment<K, V, E extends InternalEntry<K, V, E>, S extends Segment<K, V, E, S>>
            extends ReentrantLock {

        /*
         * Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so
         * can be read without locking. Next fields of nodes are immutable (final). All list
         * additions are performed at the front of each bin. This makes it easy to check changes,
         * and also fast to traverse. When nodes would otherwise be changed, new nodes are created
         * to replace them. This works well for hash tables since the bin lists tend to be short.
         * (The average length is less than two.)
         *
         * Read operations can thus proceed without locking, but rely on selected uses of volatiles
         * to ensure that completed write operations performed by other threads are noticed. For
         * most purposes, the "count" field, tracking the number of elements, serves as that
         * volatile variable ensuring visibility. This is convenient because this field needs to be
         * read in many read operations anyway:
         *
         * - All (unsynchronized) read operations must first read the "count" field, and should not
         * look at table entries if it is 0.
         *
         * - All (synchronized) write operations should write to the "count" field after
         * structurally changing any bin. The operations must not take any action that could even
         * momentarily cause a concurrent read operation to see inconsistent data. This is made
         * easier by the nature of the read operations in Map. For example, no operation can reveal
         * that the table has grown but the threshold has not yet been updated, so there are no
         * atomicity requirements for this with respect to reads.
         *
         * As a guide, all critical volatile reads and writes to the count field are marked in code
         * comments.
         */

        @Weak
        final MapMakerInternalMap<K, V, E, S> map;

        /**
         * The number of live elements in this segment's region. This does not include unset
         * elements which are awaiting cleanup.
         */
        volatile int count;

        /**
         * Number of updates that alter the size of the table. This is used during bulk-read methods
         * to make sure they see a consistent snapshot: If modCounts change during a traversal of
         * segments computing size or checking containsValue, then we might have an inconsistent
         * view of state so (usually) must retry.
         */
        int modCount;

        /**
         * The table is expanded when its size exceeds this threshold. (The value of this field is
         * always {@code (int) (capacity * 0.75)}.)
         */
        int threshold;

        /** The per-segment table. */
        volatile AtomicReferenceArray<E> table;

        /**
         * The maximum size of this map. MapMaker.UNSET_INT if there is no maximum.
         */
        final int maxSegmentSize;

        /**
         * A counter of the number of reads since the last write, used to drain queues on a small
         * fraction of read operations.
         */
        final AtomicInteger readCount = new AtomicInteger();

        Segment(MapMakerInternalMap<K, V, E, S> map, int initialCapacity, int maxSegmentSize) {
            this.map = map;
            this.maxSegmentSize = maxSegmentSize;
            initTable(newEntryArray(initialCapacity));
        }

        /**
         * Returns {@code this} up-casted to the specific {@link Segment} implementation type
         * {@code S}.
         *
         * <p>
         * This method exists so that the {@link Segment} code can be generic in terms of {@code S},
         * the type of the concrete implementation.
         */
        abstract S self();

        /** Drains the reference queues used by this segment, if any. */
        @GuardedBy("this")
        void maybeDrainReferenceQueues() {}

        /** Clears the reference queues used by this segment, if any. */
        void maybeClearReferenceQueues() {}

        /** Sets the value of the given {@code entry}. */
        void setValue(E entry, V value) {
            this.map.entryHelper.setValue(self(), entry, value);
        }

        /** Returns a copy of the given {@code entry}. */
        E copyEntry(E original, E newNext) {
            return this.map.entryHelper.copy(self(), original, newNext);
        }

        AtomicReferenceArray<E> newEntryArray(int size) {
            return new AtomicReferenceArray<E>(size);
        }

        void initTable(AtomicReferenceArray<E> newTable) {
            this.threshold = newTable.length() * 3 / 4; // 0.75
            if (this.threshold == maxSegmentSize) {
                // prevent spurious expansion before eviction
                this.threshold++;
            }
            this.table = newTable;
        }

        // Convenience methods for testing

        /**
         * Unsafe cast of the given entry to {@code E}, the type of the specific
         * {@link InternalEntry} implementation type.
         *
         * <p>
         * This method is provided as a convenience for tests. Otherwise they'd need to be
         * knowledgable about all the implementation details of our type system trickery.
         */
        abstract E castForTesting(InternalEntry<K, V, ?> entry);

        /** Unsafely extracts the key reference queue used by this segment. */
        ReferenceQueue<K> getKeyReferenceQueueForTesting() {
            throw new AssertionError();
        }

        /** Unsafely extracts the value reference queue used by this segment. */
        ReferenceQueue<V> getValueReferenceQueueForTesting() {
            throw new AssertionError();
        }

        /** Unsafely extracts the weak value reference inside of the given {@code entry}. */
        WeakValueReference<K, V, E> getWeakValueReferenceForTesting(InternalEntry<K, V, ?> entry) {
            throw new AssertionError();
        }

        /**
         * Unsafely creates of a fresh {@link WeakValueReference}, referencing the given
         * {@code value}, for the given {@code entry}
         */
        WeakValueReference<K, V, E> newWeakValueReferenceForTesting(InternalEntry<K, V, ?> entry, V value) {
            throw new AssertionError();
        }

        /**
         * Unsafely sets the weak value reference inside the given {@code entry} to be the given
         * {@code
         * valueReference}
         */
        void setWeakValueReferenceForTesting(InternalEntry<K, V, ?> entry,
                WeakValueReference<K, V, ? extends InternalEntry<K, V, ?>> valueReference) {
            throw new AssertionError();
        }

        /**
         * Unsafely sets the given index of this segment's internal hash table to be the given
         * entry.
         */
        void setTableEntryForTesting(int i, InternalEntry<K, V, ?> entry) {
            table.set(i, castForTesting(entry));
        }

        /** Unsafely returns a copy of the given entry. */
        E copyForTesting(InternalEntry<K, V, ?> entry, @Nullable InternalEntry<K, V, ?> newNext) {
            return this.map.entryHelper.copy(self(), castForTesting(entry), castForTesting(newNext));
        }

        /** Unsafely sets the value of the given entry. */
        void setValueForTesting(InternalEntry<K, V, ?> entry, V value) {
            this.map.entryHelper.setValue(self(), castForTesting(entry), value);
        }

        /** Unsafely returns a fresh entry. */
        E newEntryForTesting(K key, int hash, @Nullable InternalEntry<K, V, ?> next) {
            return this.map.entryHelper.newEntry(self(), key, hash, castForTesting(next));
        }

        /** Unsafely removes the given entry from this segment's hash table. */
        @CanIgnoreReturnValue
        boolean removeTableEntryForTesting(InternalEntry<K, V, ?> entry) {
            return removeEntryForTesting(castForTesting(entry));
        }

        /** Unsafely removes the given entry from the given chain in this segment's hash table. */
        E removeFromChainForTesting(InternalEntry<K, V, ?> first, InternalEntry<K, V, ?> entry) {
            return removeFromChain(castForTesting(first), castForTesting(entry));
        }

        /**
         * Unsafely returns the value of the given entry if it's still live, or {@code null}
         * otherwise.
         */
        @Nullable
        V getLiveValueForTesting(InternalEntry<K, V, ?> entry) {
            return getLiveValue(castForTesting(entry));
        }

        // reference queues, for garbage collection cleanup

        /**
         * Cleanup collected entries when the lock is available.
         */
        void tryDrainReferenceQueues() {
            if (tryLock()) {
                try {
                    maybeDrainReferenceQueues();
                } finally {
                    unlock();
                }
            }
        }

        @GuardedBy("this")
        void drainKeyReferenceQueue(ReferenceQueue<K> keyReferenceQueue) {
            Reference<? extends K> ref;
            int i = 0;
            while ((ref = keyReferenceQueue.poll()) != null) {
                @SuppressWarnings("unchecked")
                E entry = (E) ref;
                map.reclaimKey(entry);
                if (++i == DRAIN_MAX) {
                    break;
                }
            }
        }

        @GuardedBy("this")
        void drainValueReferenceQueue(ReferenceQueue<V> valueReferenceQueue) {
            Reference<? extends V> ref;
            int i = 0;
            while ((ref = valueReferenceQueue.poll()) != null) {
                @SuppressWarnings("unchecked")
                WeakValueReference<K, V, E> valueReference = (WeakValueReference<K, V, E>) ref;
                map.reclaimValue(valueReference);
                if (++i == DRAIN_MAX) {
                    break;
                }
            }
        }

        <T> void clearReferenceQueue(ReferenceQueue<T> referenceQueue) {
            while (referenceQueue.poll() != null) {
            }
        }

        /** Returns first entry of bin for given hash. */
        E getFirst(int hash) {
            // read this volatile field only once
            AtomicReferenceArray<E> table = this.table;
            return table.get(hash & (table.length() - 1));
        }

        // Specialized implementations of map methods

        E getEntry(Object key, int hash) {
            if (count != 0) { // read-volatile
                for (E e = getFirst(hash); e != null; e = e.getNext()) {
                    if (e.getHash() != hash) {
                        continue;
                    }

                    K entryKey = e.getKey();
                    if (entryKey == null) {
                        tryDrainReferenceQueues();
                        continue;
                    }

                    if (map.keyEquivalence.equivalent(key, entryKey)) {
                        return e;
                    }
                }
            }

            return null;
        }

        E getLiveEntry(Object key, int hash) {
            return getEntry(key, hash);
        }

        V get(Object key, int hash) {
            try {
                E e = getLiveEntry(key, hash);
                if (e == null) {
                    return null;
                }

                V value = e.getValue();
                if (value == null) {
                    tryDrainReferenceQueues();
                }
                return value;
            } finally {
                postReadCleanup();
            }
        }

        boolean containsKey(Object key, int hash) {
            try {
                if (count != 0) { // read-volatile
                    E e = getLiveEntry(key, hash);
                    return e != null && e.getValue() != null;
                }

                return false;
            } finally {
                postReadCleanup();
            }
        }

        /**
         * This method is a convenience for testing. Code should call
         * {@link MapMakerInternalMap#containsValue} directly.
         */
        @VisibleForTesting
        boolean containsValue(Object value) {
            try {
                if (count != 0) { // read-volatile
                    AtomicReferenceArray<E> table = this.table;
                    int length = table.length();
                    for (int i = 0; i < length; ++i) {
                        for (E e = table.get(i); e != null; e = e.getNext()) {
                            V entryValue = getLiveValue(e);
                            if (entryValue == null) {
                                continue;
                            }
                            if (map.valueEquivalence().equivalent(value, entryValue)) {
                                return true;
                            }
                        }
                    }
                }

                return false;
            } finally {
                postReadCleanup();
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count + 1;
                if (newCount > this.threshold) { // ensure capacity
                    expand();
                    newCount = this.count + 1;
                }

                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                // Look for an existing entry.
                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        // We found an existing entry.

                        V entryValue = e.getValue();

                        if (entryValue == null) {
                            ++modCount;
                            setValue(e, value);
                            newCount = this.count; // count remains unchanged
                            this.count = newCount; // write-volatile
                            return null;
                        } else if (onlyIfAbsent) {
                            // Mimic
                            // "if (!map.containsKey(key)) ...
                            // else return map.get(key);
                            return entryValue;
                        } else {
                            // clobber existing entry, count remains unchanged
                            ++modCount;
                            setValue(e, value);
                            return entryValue;
                        }
                    }
                }

                // Create a new entry.
                ++modCount;
                E newEntry = map.entryHelper.newEntry(self(), key, hash, first);
                setValue(newEntry, value);
                table.set(index, newEntry);
                this.count = newCount; // write-volatile
                return null;
            } finally {
                unlock();
            }
        }

        /**
         * Expands the table if possible.
         */
        @GuardedBy("this")
        void expand() {
            AtomicReferenceArray<E> oldTable = table;
            int oldCapacity = oldTable.length();
            if (oldCapacity >= MAXIMUM_CAPACITY) {
                return;
            }

            /*
             * Reclassify nodes in each list to new Map. Because we are using power-of-two
             * expansion, the elements from each bin must either stay at same index, or move with a
             * power of two offset. We eliminate unnecessary node creation by catching cases where
             * old nodes can be reused because their next fields won't change. Statistically, at the
             * default threshold, only about one-sixth of them need cloning when a table doubles.
             * The nodes they replace will be garbage collectable as soon as they are no longer
             * referenced by any reader thread that may be in the midst of traversing table right
             * now.
             */

            int newCount = count;
            AtomicReferenceArray<E> newTable = newEntryArray(oldCapacity << 1);
            threshold = newTable.length() * 3 / 4;
            int newMask = newTable.length() - 1;
            for (int oldIndex = 0; oldIndex < oldCapacity; ++oldIndex) {
                // We need to guarantee that any existing reads of old Map can
                // proceed. So we cannot yet null out each bin.
                E head = oldTable.get(oldIndex);

                if (head != null) {
                    E next = head.getNext();
                    int headIndex = head.getHash() & newMask;

                    // Single node on list
                    if (next == null) {
                        newTable.set(headIndex, head);
                    } else {
                        // Reuse the consecutive sequence of nodes with the same target
                        // index from the end of the list. tail points to the first
                        // entry in the reusable list.
                        E tail = head;
                        int tailIndex = headIndex;
                        for (E e = next; e != null; e = e.getNext()) {
                            int newIndex = e.getHash() & newMask;
                            if (newIndex != tailIndex) {
                                // The index changed. We'll need to copy the previous entry.
                                tailIndex = newIndex;
                                tail = e;
                            }
                        }
                        newTable.set(tailIndex, tail);

                        // Clone nodes leading up to the tail.
                        for (E e = head; e != tail; e = e.getNext()) {
                            int newIndex = e.getHash() & newMask;
                            E newNext = newTable.get(newIndex);
                            E newFirst = copyEntry(e, newNext);
                            if (newFirst != null) {
                                newTable.set(newIndex, newFirst);
                            } else {
                                newCount--;
                            }
                        }
                    }
                }
            }
            table = newTable;
            this.count = newCount;
        }

        boolean replace(K key, int hash, V oldValue, V newValue) {
            lock();
            try {
                preWriteCleanup();

                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        // If the value disappeared, this entry is partially collected,
                        // and we should pretend like it doesn't exist.
                        V entryValue = e.getValue();
                        if (entryValue == null) {
                            if (isCollected(e)) {
                                int newCount = this.count - 1;
                                ++modCount;
                                E newFirst = removeFromChain(first, e);
                                newCount = this.count - 1;
                                table.set(index, newFirst);
                                this.count = newCount; // write-volatile
                            }
                            return false;
                        }

                        if (map.valueEquivalence().equivalent(oldValue, entryValue)) {
                            ++modCount;
                            setValue(e, newValue);
                            return true;
                        } else {
                            // Mimic
                            // "if (map.containsKey(key) && map.get(key).equals(oldValue))..."
                            return false;
                        }
                    }
                }

                return false;
            } finally {
                unlock();
            }
        }

        V replace(K key, int hash, V newValue) {
            lock();
            try {
                preWriteCleanup();

                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        // If the value disappeared, this entry is partially collected,
                        // and we should pretend like it doesn't exist.
                        V entryValue = e.getValue();
                        if (entryValue == null) {
                            if (isCollected(e)) {
                                int newCount = this.count - 1;
                                ++modCount;
                                E newFirst = removeFromChain(first, e);
                                newCount = this.count - 1;
                                table.set(index, newFirst);
                                this.count = newCount; // write-volatile
                            }
                            return null;
                        }

                        ++modCount;
                        setValue(e, newValue);
                        return entryValue;
                    }
                }

                return null;
            } finally {
                unlock();
            }
        }

        @CanIgnoreReturnValue
        V remove(Object key, int hash) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count - 1;
                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        V entryValue = e.getValue();

                        if (entryValue != null) {
                            // TODO(kak): Remove this branch
                        } else if (isCollected(e)) {
                            // TODO(kak): Remove this branch
                        } else {
                            return null;
                        }

                        ++modCount;
                        E newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return entryValue;
                    }
                }

                return null;
            } finally {
                unlock();
            }
        }

        boolean remove(Object key, int hash, Object value) {
            lock();
            try {
                preWriteCleanup();

                int newCount = this.count - 1;
                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        V entryValue = e.getValue();

                        boolean explicitRemoval = false;
                        if (map.valueEquivalence().equivalent(value, entryValue)) {
                            explicitRemoval = true;
                        } else if (isCollected(e)) {
                            // TODO(kak): Remove this branch
                        } else {
                            return false;
                        }

                        ++modCount;
                        E newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return explicitRemoval;
                    }
                }

                return false;
            } finally {
                unlock();
            }
        }

        void clear() {
            if (count != 0) {
                lock();
                try {
                    AtomicReferenceArray<E> table = this.table;
                    for (int i = 0; i < table.length(); ++i) {
                        table.set(i, null);
                    }
                    maybeClearReferenceQueues();
                    readCount.set(0);

                    ++modCount;
                    count = 0; // write-volatile
                } finally {
                    unlock();
                }
            }
        }

        /**
         * Removes an entry from within a table. All entries following the removed node can stay,
         * but all preceding ones need to be cloned.
         *
         * <p>
         * This method does not decrement count for the removed entry, but does decrement count for
         * all partially collected entries which are skipped. As such callers which are modifying
         * count must re-read it after calling removeFromChain.
         *
         * @param first the first entry of the table
         * @param entry the entry being removed from the table
         * @return the new first entry for the table
         */
        @GuardedBy("this")
        E removeFromChain(E first, E entry) {
            int newCount = count;
            E newFirst = entry.getNext();
            for (E e = first; e != entry; e = e.getNext()) {
                E next = copyEntry(e, newFirst);
                if (next != null) {
                    newFirst = next;
                } else {
                    newCount--;
                }
            }
            this.count = newCount;
            return newFirst;
        }

        /** Removes an entry whose key has been garbage collected. */
        @CanIgnoreReturnValue
        boolean reclaimKey(E entry, int hash) {
            lock();
            try {
                int newCount = count - 1;
                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    if (e == entry) {
                        ++modCount;
                        E newFirst = removeFromChain(first, e);
                        newCount = this.count - 1;
                        table.set(index, newFirst);
                        this.count = newCount; // write-volatile
                        return true;
                    }
                }

                return false;
            } finally {
                unlock();
            }
        }

        /** Removes an entry whose value has been garbage collected. */
        @CanIgnoreReturnValue
        boolean reclaimValue(K key, int hash, WeakValueReference<K, V, E> valueReference) {
            lock();
            try {
                int newCount = this.count - 1;
                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        WeakValueReference<K, V, E> v = ((WeakValueEntry<K, V, E>) e).getValueReference();
                        if (v == valueReference) {
                            ++modCount;
                            E newFirst = removeFromChain(first, e);
                            newCount = this.count - 1;
                            table.set(index, newFirst);
                            this.count = newCount; // write-volatile
                            return true;
                        }
                        return false;
                    }
                }

                return false;
            } finally {
                unlock();
            }
        }

        /**
         * Clears a value that has not yet been set, and thus does not require count to be modified.
         */
        @CanIgnoreReturnValue
        boolean clearValueForTesting(K key, int hash,
                WeakValueReference<K, V, ? extends InternalEntry<K, V, ?>> valueReference) {
            lock();
            try {
                AtomicReferenceArray<E> table = this.table;
                int index = hash & (table.length() - 1);
                E first = table.get(index);

                for (E e = first; e != null; e = e.getNext()) {
                    K entryKey = e.getKey();
                    if (e.getHash() == hash && entryKey != null && map.keyEquivalence.equivalent(key, entryKey)) {
                        WeakValueReference<K, V, E> v = ((WeakValueEntry<K, V, E>) e).getValueReference();
                        if (v == valueReference) {
                            E newFirst = removeFromChain(first, e);
                            table.set(index, newFirst);
                            return true;
                        }
                        return false;
                    }
                }

                return false;
            } finally {
                unlock();
            }
        }

        @GuardedBy("this")
        boolean removeEntryForTesting(E entry) {
            int hash = entry.getHash();
            int newCount = this.count - 1;
            AtomicReferenceArray<E> table = this.table;
            int index = hash & (table.length() - 1);
            E first = table.get(index);

            for (E e = first; e != null; e = e.getNext()) {
                if (e == entry) {
                    ++modCount;
                    E newFirst = removeFromChain(first, e);
                    newCount = this.count - 1;
                    table.set(index, newFirst);
                    this.count = newCount; // write-volatile
                    return true;
                }
            }

            return false;
        }

        /**
         * Returns {@code true} if the value has been partially collected, meaning that the value is
         * null.
         */
        static <K, V, E extends InternalEntry<K, V, E>> boolean isCollected(E entry) {
            return entry.getValue() == null;
        }

        /**
         * Gets the value from an entry. Returns {@code null} if the entry is invalid or
         * partially-collected.
         */
        @Nullable
        V getLiveValue(E entry) {
            if (entry.getKey() == null) {
                tryDrainReferenceQueues();
                return null;
            }
            V value = entry.getValue();
            if (value == null) {
                tryDrainReferenceQueues();
                return null;
            }

            return value;
        }

        /**
         * Performs routine cleanup following a read. Normally cleanup happens during writes, or
         * from the cleanupExecutor. If cleanup is not observed after a sufficient number of reads,
         * try cleaning up from the read thread.
         */
        void postReadCleanup() {
            if ((readCount.incrementAndGet() & DRAIN_THRESHOLD) == 0) {
                runCleanup();
            }
        }

        /**
         * Performs routine cleanup prior to executing a write. This should be called every time a
         * write thread acquires the segment lock, immediately after acquiring the lock.
         */
        @GuardedBy("this")
        void preWriteCleanup() {
            runLockedCleanup();
        }

        void runCleanup() {
            runLockedCleanup();
        }

        void runLockedCleanup() {
            if (tryLock()) {
                try {
                    maybeDrainReferenceQueues();
                    readCount.set(0);
                } finally {
                    unlock();
                }
            }
        }
    }

    /** Concrete implementation of {@link Segment} for strong keys and strong values. */
    static final class StrongKeyStrongValueSegment<K, V>
            extends Segment<K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>> {
        StrongKeyStrongValueSegment(
                MapMakerInternalMap<K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>> map,
                int initialCapacity, int maxSegmentSize) {
            super(map, initialCapacity, maxSegmentSize);
        }

        @Override
        StrongKeyStrongValueSegment<K, V> self() {
            return this;
        }

        @SuppressWarnings("unchecked")
        @Override
        public StrongKeyStrongValueEntry<K, V> castForTesting(InternalEntry<K, V, ?> entry) {
            return (StrongKeyStrongValueEntry<K, V>) entry;
        }
    }

    /** Concrete implementation of {@link Segment} for strong keys and weak values. */
    static final class StrongKeyWeakValueSegment<K, V>
            extends Segment<K, V, StrongKeyWeakValueEntry<K, V>, StrongKeyWeakValueSegment<K, V>> {
        private final ReferenceQueue<V> queueForValues = new ReferenceQueue<V>();

        StrongKeyWeakValueSegment(
                MapMakerInternalMap<K, V, StrongKeyWeakValueEntry<K, V>, StrongKeyWeakValueSegment<K, V>> map,
                int initialCapacity, int maxSegmentSize) {
            super(map, initialCapacity, maxSegmentSize);
        }

        @Override
        StrongKeyWeakValueSegment<K, V> self() {
            return this;
        }

        @Override
        ReferenceQueue<V> getValueReferenceQueueForTesting() {
            return queueForValues;
        }

        @SuppressWarnings("unchecked")
        @Override
        public StrongKeyWeakValueEntry<K, V> castForTesting(InternalEntry<K, V, ?> entry) {
            return (StrongKeyWeakValueEntry<K, V>) entry;
        }

        @Override
        public WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> getWeakValueReferenceForTesting(
                InternalEntry<K, V, ?> e) {
            return castForTesting(e).getValueReference();
        }

        @Override
        public WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> newWeakValueReferenceForTesting(
                InternalEntry<K, V, ?> e, V value) {
            return new WeakValueReferenceImpl<K, V, StrongKeyWeakValueEntry<K, V>>(queueForValues, value,
                    castForTesting(e));
        }

        @Override
        public void setWeakValueReferenceForTesting(InternalEntry<K, V, ?> e,
                WeakValueReference<K, V, ? extends InternalEntry<K, V, ?>> valueReference) {
            StrongKeyWeakValueEntry<K, V> entry = castForTesting(e);
            @SuppressWarnings("unchecked")
            WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> newValueReference =
                    (WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>>) valueReference;
            WeakValueReference<K, V, StrongKeyWeakValueEntry<K, V>> previous = entry.valueReference;
            entry.valueReference = newValueReference;
            previous.clear();
        }

        @Override
        void maybeDrainReferenceQueues() {
            drainValueReferenceQueue(queueForValues);
        }

        @Override
        void maybeClearReferenceQueues() {
            clearReferenceQueue(queueForValues);
        }
    }

    /** Concrete implementation of {@link Segment} for weak keys and strong values. */
    static final class WeakKeyStrongValueSegment<K, V>
            extends Segment<K, V, WeakKeyStrongValueEntry<K, V>, WeakKeyStrongValueSegment<K, V>> {
        private final ReferenceQueue<K> queueForKeys = new ReferenceQueue<K>();

        WeakKeyStrongValueSegment(
                MapMakerInternalMap<K, V, WeakKeyStrongValueEntry<K, V>, WeakKeyStrongValueSegment<K, V>> map,
                int initialCapacity, int maxSegmentSize) {
            super(map, initialCapacity, maxSegmentSize);
        }

        @Override
        WeakKeyStrongValueSegment<K, V> self() {
            return this;
        }

        @Override
        ReferenceQueue<K> getKeyReferenceQueueForTesting() {
            return queueForKeys;
        }

        @SuppressWarnings("unchecked")
        @Override
        public WeakKeyStrongValueEntry<K, V> castForTesting(InternalEntry<K, V, ?> entry) {
            return (WeakKeyStrongValueEntry<K, V>) entry;
        }

        @Override
        void maybeDrainReferenceQueues() {
            drainKeyReferenceQueue(queueForKeys);
        }

        @Override
        void maybeClearReferenceQueues() {
            clearReferenceQueue(queueForKeys);
        }
    }

    /** Concrete implementation of {@link Segment} for weak keys and weak values. */
    static final class WeakKeyWeakValueSegment<K, V>
            extends Segment<K, V, WeakKeyWeakValueEntry<K, V>, WeakKeyWeakValueSegment<K, V>> {
        private final ReferenceQueue<K> queueForKeys = new ReferenceQueue<K>();
        private final ReferenceQueue<V> queueForValues = new ReferenceQueue<V>();

        WeakKeyWeakValueSegment(
                MapMakerInternalMap<K, V, WeakKeyWeakValueEntry<K, V>, WeakKeyWeakValueSegment<K, V>> map,
                int initialCapacity, int maxSegmentSize) {
            super(map, initialCapacity, maxSegmentSize);
        }

        @Override
        WeakKeyWeakValueSegment<K, V> self() {
            return this;
        }

        @Override
        ReferenceQueue<K> getKeyReferenceQueueForTesting() {
            return queueForKeys;
        }

        @Override
        ReferenceQueue<V> getValueReferenceQueueForTesting() {
            return queueForValues;
        }

        @SuppressWarnings("unchecked")
        @Override
        public WeakKeyWeakValueEntry<K, V> castForTesting(InternalEntry<K, V, ?> entry) {
            return (WeakKeyWeakValueEntry<K, V>) entry;
        }

        @Override
        public WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> getWeakValueReferenceForTesting(
                InternalEntry<K, V, ?> e) {
            return (castForTesting(e)).getValueReference();
        }

        @Override
        public WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> newWeakValueReferenceForTesting(
                InternalEntry<K, V, ?> e, V value) {
            return new WeakValueReferenceImpl<K, V, WeakKeyWeakValueEntry<K, V>>(queueForValues, value,
                    castForTesting(e));
        }

        @Override
        public void setWeakValueReferenceForTesting(InternalEntry<K, V, ?> e,
                WeakValueReference<K, V, ? extends InternalEntry<K, V, ?>> valueReference) {
            WeakKeyWeakValueEntry<K, V> entry = castForTesting(e);
            @SuppressWarnings("unchecked")
            WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> newValueReference =
                    (WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>>) valueReference;
            WeakValueReference<K, V, WeakKeyWeakValueEntry<K, V>> previous = entry.valueReference;
            entry.valueReference = newValueReference;
            previous.clear();
        }

        @Override
        void maybeDrainReferenceQueues() {
            drainKeyReferenceQueue(queueForKeys);
            drainValueReferenceQueue(queueForValues);
        }

        @Override
        void maybeClearReferenceQueues() {
            clearReferenceQueue(queueForKeys);
        }
    }

    static final class CleanupMapTask implements Runnable {
        final WeakReference<MapMakerInternalMap<?, ?, ?, ?>> mapReference;

        public CleanupMapTask(MapMakerInternalMap<?, ?, ?, ?> map) {
            this.mapReference = new WeakReference<MapMakerInternalMap<?, ?, ?, ?>>(map);
        }

        @Override
        public void run() {
            MapMakerInternalMap<?, ?, ?, ?> map = mapReference.get();
            if (map == null) {
                throw new CancellationException();
            }

            for (Segment<?, ?, ?, ?> segment : map.segments) {
                segment.runCleanup();
            }
        }
    }

    @VisibleForTesting
    Strength keyStrength() {
        return entryHelper.keyStrength();
    }

    @VisibleForTesting
    Strength valueStrength() {
        return entryHelper.valueStrength();
    }

    @VisibleForTesting
    Equivalence<Object> valueEquivalence() {
        return entryHelper.valueStrength().defaultEquivalence();
    }

    // ConcurrentMap methods

    @Override
    public boolean isEmpty() {
        /*
         * Sum per-segment modCounts to avoid mis-reporting when elements are concurrently added and
         * removed in one segment while checking another, in which case the table was never actually
         * empty at any point. (The sum ensures accuracy up through at least 1<<31 per-segment
         * modifications before recheck.) Method containsValue() uses similar constructions for
         * stability checks.
         */
        long sum = 0L;
        Segment<K, V, E, S>[] segments = this.segments;
        for (int i = 0; i < segments.length; ++i) {
            if (segments[i].count != 0) {
                return false;
            }
            sum += segments[i].modCount;
        }

        if (sum != 0L) { // recheck unless no modifications
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].count != 0) {
                    return false;
                }
                sum -= segments[i].modCount;
            }
            if (sum != 0L) {
                return false;
            }
        }
        return true;
    }

    @Override
    public int size() {
        Segment<K, V, E, S>[] segments = this.segments;
        long sum = 0;
        for (int i = 0; i < segments.length; ++i) {
            sum += segments[i].count;
        }
        return Ints.saturatedCast(sum);
    }

    @Override
    public V get(@Nullable Object key) {
        if (key == null) {
            return null;
        }
        int hash = hash(key);
        return segmentFor(hash).get(key, hash);
    }

    /**
     * Returns the internal entry for the specified key. The entry may be computing or partially
     * collected. Does not impact recency ordering.
     */
    E getEntry(@Nullable Object key) {
        if (key == null) {
            return null;
        }
        int hash = hash(key);
        return segmentFor(hash).getEntry(key, hash);
    }

    @Override
    public boolean containsKey(@Nullable Object key) {
        if (key == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).containsKey(key, hash);
    }

    @Override
    public boolean containsValue(@Nullable Object value) {
        if (value == null) {
            return false;
        }

        // This implementation is patterned after ConcurrentHashMap, but without the locking. The
        // only
        // way for it to return a false negative would be for the target value to jump around in the
        // map
        // such that none of the subsequent iterations observed it, despite the fact that at every
        // point
        // in time it was present somewhere int the map. This becomes increasingly unlikely as
        // CONTAINS_VALUE_RETRIES increases, though without locking it is theoretically possible.
        final Segment<K, V, E, S>[] segments = this.segments;
        long last = -1L;
        for (int i = 0; i < CONTAINS_VALUE_RETRIES; i++) {
            long sum = 0L;
            for (Segment<K, V, E, S> segment : segments) {
                // ensure visibility of most recent completed write
                int unused = segment.count; // read-volatile

                AtomicReferenceArray<E> table = segment.table;
                for (int j = 0; j < table.length(); j++) {
                    for (E e = table.get(j); e != null; e = e.getNext()) {
                        V v = segment.getLiveValue(e);
                        if (v != null && valueEquivalence().equivalent(value, v)) {
                            return true;
                        }
                    }
                }
                sum += segment.modCount;
            }
            if (sum == last) {
                break;
            }
            last = sum;
        }
        return false;
    }

    @CanIgnoreReturnValue
    @Override
    public V put(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).put(key, hash, value, false);
    }

    @CanIgnoreReturnValue
    @Override
    public V putIfAbsent(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).put(key, hash, value, true);
    }

    @Override
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Entry<? extends K, ? extends V> e : m.entrySet()) {
            put(e.getKey(), e.getValue());
        }
    }

    @CanIgnoreReturnValue
    @Override
    public V remove(@Nullable Object key) {
        if (key == null) {
            return null;
        }
        int hash = hash(key);
        return segmentFor(hash).remove(key, hash);
    }

    @CanIgnoreReturnValue
    @Override
    public boolean remove(@Nullable Object key, @Nullable Object value) {
        if (key == null || value == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).remove(key, hash, value);
    }

    @CanIgnoreReturnValue
    @Override
    public boolean replace(K key, @Nullable V oldValue, V newValue) {
        checkNotNull(key);
        checkNotNull(newValue);
        if (oldValue == null) {
            return false;
        }
        int hash = hash(key);
        return segmentFor(hash).replace(key, hash, oldValue, newValue);
    }

    @CanIgnoreReturnValue
    @Override
    public V replace(K key, V value) {
        checkNotNull(key);
        checkNotNull(value);
        int hash = hash(key);
        return segmentFor(hash).replace(key, hash, value);
    }

    @Override
    public void clear() {
        for (Segment<K, V, E, S> segment : segments) {
            segment.clear();
        }
    }

    transient Set<K> keySet;

    @Override
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    transient Collection<V> values;

    @Override
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    transient Set<Entry<K, V>> entrySet;

    @Override
    public Set<Entry<K, V>> entrySet() {
        Set<Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    // Iterator Support

    abstract class HashIterator<T> implements Iterator<T> {

        int nextSegmentIndex;
        int nextTableIndex;
        Segment<K, V, E, S> currentSegment;
        AtomicReferenceArray<E> currentTable;
        E nextEntry;
        WriteThroughEntry nextExternal;
        WriteThroughEntry lastReturned;

        HashIterator() {
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        @Override
        public abstract T next();

        final void advance() {
            nextExternal = null;

            if (nextInChain()) {
                return;
            }

            if (nextInTable()) {
                return;
            }

            while (nextSegmentIndex >= 0) {
                currentSegment = segments[nextSegmentIndex--];
                if (currentSegment.count != 0) {
                    currentTable = currentSegment.table;
                    nextTableIndex = currentTable.length() - 1;
                    if (nextInTable()) {
                        return;
                    }
                }
            }
        }

        /**
         * Finds the next entry in the current chain. Returns {@code true} if an entry was found.
         */
        boolean nextInChain() {
            if (nextEntry != null) {
                for (nextEntry = nextEntry.getNext(); nextEntry != null; nextEntry = nextEntry.getNext()) {
                    if (advanceTo(nextEntry)) {
                        return true;
                    }
                }
            }
            return false;
        }

        /**
         * Finds the next entry in the current table. Returns {@code true} if an entry was found.
         */
        boolean nextInTable() {
            while (nextTableIndex >= 0) {
                if ((nextEntry = currentTable.get(nextTableIndex--)) != null) {
                    if (advanceTo(nextEntry) || nextInChain()) {
                        return true;
                    }
                }
            }
            return false;
        }

        /**
         * Advances to the given entry. Returns {@code true} if the entry was valid, {@code false}
         * if it should be skipped.
         */
        boolean advanceTo(E entry) {
            try {
                K key = entry.getKey();
                V value = getLiveValue(entry);
                if (value != null) {
                    nextExternal = new WriteThroughEntry(key, value);
                    return true;
                } else {
                    // Skip stale entry.
                    return false;
                }
            } finally {
                currentSegment.postReadCleanup();
            }
        }

        @Override
        public boolean hasNext() {
            return nextExternal != null;
        }

        WriteThroughEntry nextEntry() {
            if (nextExternal == null) {
                throw new NoSuchElementException();
            }
            lastReturned = nextExternal;
            advance();
            return lastReturned;
        }

        @Override
        public void remove() {
            checkRemove(lastReturned != null);
            MapMakerInternalMap.this.remove(lastReturned.getKey());
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator<K> {

        @Override
        public K next() {
            return nextEntry().getKey();
        }
    }

    final class ValueIterator extends HashIterator<V> {

        @Override
        public V next() {
            return nextEntry().getValue();
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays setValue changes to the
     * underlying map.
     */
    final class WriteThroughEntry extends AbstractMapEntry<K, V> {
        final K key; // non-null
        V value; // non-null

        WriteThroughEntry(K key, V value) {
            this.key = key;
            this.value = value;
        }

        @Override
        public K getKey() {
            return key;
        }

        @Override
        public V getValue() {
            return value;
        }

        @Override
        public boolean equals(@Nullable Object object) {
            // Cannot use key and value equivalence
            if (object instanceof Entry) {
                Entry<?, ?> that = (Entry<?, ?>) object;
                return key.equals(that.getKey()) && value.equals(that.getValue());
            }
            return false;
        }

        @Override
        public int hashCode() {
            // Cannot use key and value equivalence
            return key.hashCode() ^ value.hashCode();
        }

        @Override
        public V setValue(V newValue) {
            V oldValue = put(key, newValue);
            value = newValue; // only if put succeeds
            return oldValue;
        }
    }

    final class EntryIterator extends HashIterator<Entry<K, V>> {

        @Override
        public Entry<K, V> next() {
            return nextEntry();
        }
    }

    @WeakOuter
    final class KeySet extends SafeToArraySet<K> {

        @Override
        public Iterator<K> iterator() {
            return new KeyIterator();
        }

        @Override
        public int size() {
            return MapMakerInternalMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return MapMakerInternalMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o) {
            return MapMakerInternalMap.this.containsKey(o);
        }

        @Override
        public boolean remove(Object o) {
            return MapMakerInternalMap.this.remove(o) != null;
        }

        @Override
        public void clear() {
            MapMakerInternalMap.this.clear();
        }
    }

    @WeakOuter
    final class Values extends AbstractCollection<V> {

        @Override
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        @Override
        public int size() {
            return MapMakerInternalMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return MapMakerInternalMap.this.isEmpty();
        }

        @Override
        public boolean contains(Object o) {
            return MapMakerInternalMap.this.containsValue(o);
        }

        @Override
        public void clear() {
            MapMakerInternalMap.this.clear();
        }

        // super.toArray() may misbehave if size() is inaccurate, at least on old versions of
        // Android.
        // https://code.google.com/p/android/issues/detail?id=36519 / http://r.android.com/47508

        @Override
        public Object[] toArray() {
            return toArrayList(this).toArray();
        }

        @Override
        public <E> E[] toArray(E[] a) {
            return toArrayList(this).toArray(a);
        }
    }

    @WeakOuter
    final class EntrySet extends SafeToArraySet<Entry<K, V>> {

        @Override
        public Iterator<Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        @Override
        public boolean contains(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry<?, ?> e = (Entry<?, ?>) o;
            Object key = e.getKey();
            if (key == null) {
                return false;
            }
            V v = MapMakerInternalMap.this.get(key);

            return v != null && valueEquivalence().equivalent(e.getValue(), v);
        }

        @Override
        public boolean remove(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry<?, ?> e = (Entry<?, ?>) o;
            Object key = e.getKey();
            return key != null && MapMakerInternalMap.this.remove(key, e.getValue());
        }

        @Override
        public int size() {
            return MapMakerInternalMap.this.size();
        }

        @Override
        public boolean isEmpty() {
            return MapMakerInternalMap.this.isEmpty();
        }

        @Override
        public void clear() {
            MapMakerInternalMap.this.clear();
        }
    }

    private abstract static class SafeToArraySet<E> extends AbstractSet<E> {
        // super.toArray() may misbehave if size() is inaccurate, at least on old versions of
        // Android.
        // https://code.google.com/p/android/issues/detail?id=36519 / http://r.android.com/47508

        @Override
        public Object[] toArray() {
            return toArrayList(this).toArray();
        }

        @Override
        public <E> E[] toArray(E[] a) {
            return toArrayList(this).toArray(a);
        }
    }

    private static <E> ArrayList<E> toArrayList(Collection<E> c) {
        // Avoid calling ArrayList(Collection), which may call back into toArray.
        ArrayList<E> result = new ArrayList<E>(c.size());
        Iterators.addAll(result, c.iterator());
        return result;
    }

    // Serialization Support

    private static final long serialVersionUID = 5;

    Object writeReplace() {
        return new SerializationProxy<K, V>(entryHelper.keyStrength(), entryHelper.valueStrength(), keyEquivalence,
                entryHelper.valueStrength().defaultEquivalence(), concurrencyLevel, this);
    }

    /**
     * The actual object that gets serialized. Unfortunately, readResolve() doesn't get called when
     * a circular dependency is present, so the proxy must be able to behave as the map itself.
     */
    abstract static class AbstractSerializationProxy<K, V> extends ForwardingConcurrentMap<K, V>
            implements Serializable {
        private static final long serialVersionUID = 3;

        final Strength keyStrength;
        final Strength valueStrength;
        final Equivalence<Object> keyEquivalence;
        final Equivalence<Object> valueEquivalence;
        final int concurrencyLevel;

        transient ConcurrentMap<K, V> delegate;

        AbstractSerializationProxy(Strength keyStrength, Strength valueStrength, Equivalence<Object> keyEquivalence,
                Equivalence<Object> valueEquivalence, int concurrencyLevel, ConcurrentMap<K, V> delegate) {
            this.keyStrength = keyStrength;
            this.valueStrength = valueStrength;
            this.keyEquivalence = keyEquivalence;
            this.valueEquivalence = valueEquivalence;
            this.concurrencyLevel = concurrencyLevel;
            this.delegate = delegate;
        }

        @Override
        protected ConcurrentMap<K, V> delegate() {
            return delegate;
        }

        void writeMapTo(ObjectOutputStream out) throws IOException {
            out.writeInt(delegate.size());
            for (Entry<K, V> entry : delegate.entrySet()) {
                out.writeObject(entry.getKey());
                out.writeObject(entry.getValue());
            }
            out.writeObject(null); // terminate entries
        }

        @SuppressWarnings("deprecation") // serialization of deprecated feature
        MapMaker readMapMaker(ObjectInputStream in) throws IOException {
            int size = in.readInt();
            return new MapMaker().initialCapacity(size).setKeyStrength(keyStrength).setValueStrength(valueStrength)
                    .keyEquivalence(keyEquivalence).concurrencyLevel(concurrencyLevel);
        }

        @SuppressWarnings("unchecked")
        void readEntries(ObjectInputStream in) throws IOException, ClassNotFoundException {
            while (true) {
                K key = (K) in.readObject();
                if (key == null) {
                    break; // terminator
                }
                V value = (V) in.readObject();
                delegate.put(key, value);
            }
        }
    }

    /**
     * The actual object that gets serialized. Unfortunately, readResolve() doesn't get called when
     * a circular dependency is present, so the proxy must be able to behave as the map itself.
     */
    private static final class SerializationProxy<K, V> extends AbstractSerializationProxy<K, V> {
        private static final long serialVersionUID = 3;

        SerializationProxy(Strength keyStrength, Strength valueStrength, Equivalence<Object> keyEquivalence,
                Equivalence<Object> valueEquivalence, int concurrencyLevel, ConcurrentMap<K, V> delegate) {
            super(keyStrength, valueStrength, keyEquivalence, valueEquivalence, concurrencyLevel, delegate);
        }

        private void writeObject(ObjectOutputStream out) throws IOException {
            out.defaultWriteObject();
            writeMapTo(out);
        }

        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            MapMaker mapMaker = readMapMaker(in);
            delegate = mapMaker.makeMap();
            readEntries(in);
        }

        private Object readResolve() {
            return delegate;
        }
    }
}
